Integrated device with a surface electrical field delimiting structure and relative fabrication process

Abstract: The resin sealing layer (30) enclosing the device (10) is biased to a low voltage by means of an anchoring structure (32) formed close to high-voltage contact pads (23). The anchoring structure is formed by a metal region (24) deposited on the surface of the device (10) and contacting the resin layer (30), and by a deep region (28) extending from the surface (19) of the device, beneath the metal region (24), to the substrate (11). The electrical field in the resin layer (30) is confined between the high-voltage pads (23) and the anchoring structure (32) and prevented from generating polarity inversions in the semiconductor material at the low-voltage contact pads (25) or any other points at which the resin layer (30) contacts the body of semiconductor material (11, 12).

Description: The present invention relates to an integrated device with a surface electrical field delimiting structure, and relative fabrication process, according to the preamble of claims 1 and 5. As is known, integrated devices frequently present surface electrical fields caused by charges in the surface layers of the device or by surface layers biased to given potentials, which surface fields bias the surface layers and/or the overall protective surface layer of the device. Surface electrical fields are undesired on account of possibly inverting the polarity of certain surface portions of the device, thus forming, and possibly also turning on, parasitic MOS transistors and so impairing operation of the device. This possibility is typically encountered in the case of high-voltage devices (e.g. up to 600 V) with high-voltage contact pads left exposed by the passivation layer interposed between the surface of the device and the protective layer. In this case, the high-voltage pads directly contacting the protective layer bias this to their own high potential and may possibly invert the surface layer of the adjacent body of semiconductor material. To demonstrate this more clearly, an example structure is illustrated in Figure 2 which shows the surface portion 1 of a high-voltage integrated device 2; a passivation layer 3 covering surface 4 of device 2; and an overall plastic protective layer 5. At various points, passivation layer 3 is interrupted by contact pads, of which Figure 2 shows only a high-voltage pad 6 and a low-voltage pad 7. In the example shown, the electrical field generated by high-voltage pad 6 biases protective layer 5 and produces a difference in potential in the portion between pads 6 and 7; and, if the voltage of pad 6 is high, biasing of protective layer 5 may invert the polarity of the underlying portion 8 of the device. Currently used techniques for eliminating or at least delimiting surface electrical fields comprise the formation of field plates or channel stoppers, which structures, however, take up a considerable area if formed over the entire surface of the device. An integrated device of the type defined in the preamble of claim 1 is disclosed in GB-A-1 251 456, wherein for causing the performance of the device not being rendered erratic due to ion migration between the plastics material and a passivating layer, metal shields are formed in the passivating layer over PN junctions, the metal shield being connected to a portion of the semiconductor body forming the PN junction so as to be at the same potential as that semiconductor body portion. EP-A-0 567 694 describes an integrated semiconductor device formed by a stack of semiconductor circuit blocks superimposed to each other and comprising vertical shielding formations for shielding noise, e.g. clock or switching noise. It is an object of the present invention to provide an integrated device with a structure capable of delimiting the electrical fields in the protective layer without recourse to structures such as field plates or channel stoppers. According to the present invention, there is provided an integrated device with a surface electrical field delimiting structure, as claimed in Claim 1, and a process for fabricating such a device, as claimed in Claim 9. In practice, according to the present invention, the protective layer enclosing the device is biased to a low voltage so that the electrical field in the protective layer, at the active regions implemented in the semiconductor material, presents a zero value. A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a cross section of a portion of semiconductor material implementing the integrated device with a delimiting structure according to the present invention; Figure 2 shows a cross section of a known integrated device. Of integrated device 10 in Figure 1, only the parts pertinent to the present invention are shown. More specifically, device 10 comprises a substrate 11, in this case a P type; and an epitaxial layer 12, in this case an N type. Epitaxial layer 12 presents deep regions 13, in this case P types, constituting junction isolating regions and laterally defining active regions 14 (only one shown) in which the device components are formed, e.g. bipolar or MOS transistors, diodes, etc., as shown by the electric symbols. A passivation layer 18 covers the whole of upper surface 19 of epitaxial layer 12, with the exception of openings 20, 21, 22 at respective metal portions 23, 24, 25. More specifically, metal portion 23 forms a high-voltage (e.g. 600 V) contact pad and connects (over metal lines not shown) high-voltage portions of device 10 to contact wires (not shown) for connection to pins (not shown) on the device; metal portion 25 forms a low-voltage contact pad and connects (over metal lines not shown) low-voltage portions of device 10 to respective contact wires; and metal portion 24 - located close to high-voltage pad 23 and at any rate between pad 23 and active regions 14 in which the components of device 10 are formed - is connected to substrate 11 via a deep region 28, in this case a P type formed using the so-called "top-bottom" technique, i.e. by predepositing doping agents on substrate 11, before growing epitaxial layer 12, to form the bottom portion, and by downward diffusing doping agents into the grown epitaxial layer 12 to form the top portion. The device is sealed by a protective plastic (resin) layer 30 over its entire surface. In practice, in the Figure 1 structure, passivation layer 18 is opened adjacent to high-voltage pad 23, and provision is made at opening 21 for a metal line or region 24 anchored to ground (substrate 11) via deep region 28. As such, the potential gradient in protective layer 30 is confined to the region between high-voltage pad 23 and opening 21, so that the electrical field in protective layer 30 between openings 21 and 22 is zero and prevented from inverting polarity in the active regions (such as 14) between the ground anchoring openings and low-voltage pads and so impairing operation of the integrated components in the active regions. In other words, metal portion 24 and deep region 28 form an element 32 for anchoring resin layer 30 to ground. Voltage anchoring structure 32 is preferably so formed as to surround the high-voltage surface regions contacting protective layer 30, and so separate them electrically from the low-voltage regions. It should be pointed out that the term "low voltage" is here intended to mean any potential (with respect to the minimum potential of the device, normally that of the substrate) below the value at which the semiconductor material may undergo polarity inversion. (This value, known to those skilled in the art, varies according to the process and the characteristics of the material, and, using the technology employed by the Applicant, for example, corresponds substantially to voltages below 20 V). Conversely, the term "high voltage" is intended to mean a potential of such a value as to cause polarity inversion of the semiconductor material. Moreover, anchoring structure 32 is advantageously not connected directly to any active region 14 of the device; the term "direct connection" here being intended to mean any path along which humidity can travel, and therefore excluding any electrical paths through the epitaxial layer and the substrate. Anchoring structure 32 in no way impairs the electrical characteristics of the device. In the event that humidity may infiltrate through the opening in passivation layer 18 and so corrode the underlying metal region 24, the metal region 24 is not connected to the active areas, as above indicated, therefore any corrosion of the metal region 24 does not impair operation of the device, simply reduces the field delimiting effect of the anchoring structure. Conversely, in the event the device is adequately safeguarded against humidity penetrating through opening 21, e.g. by the superior characteristics of the outer protective layer, direct electrical connections (via metal lines extending over surface 19) may be made to components of the device. The structure according to the present invention is fabricated as follows. Firstly, deep region/s 28 for anchorage to substrate 11 is/are grown using, for example, the top-bottom technique described above, and preferably simultaneously with the formation of other deep regions such as isolating regions 13. The components are then formed in epitaxial layer 12 according to the structure and characteristics of device 10, and one or more metal lines 24 (or even appropriately shaped regions) are deposited and shaped close to high-voltage pads 23. This step may also be performed simultaneously with formation of the metal contact regions and contact pads 23, 25 of the device (or of the lower metal level if the device comprises a number of metal levels). At this point, passivation layer 18 is deposited and etched to form openings 21 at metal lines or regions 24, which step may also be performed simultaneously with the formation of openings 20, 22 at contact pads 23, 25. This is then followed by the usual finish steps, including cutting the chip; bonding it to the lead frame; soldering the contact wires; and molding the resin sealing casing 30. At this stage the resin penetrates inside the passivation openings, including opening/s 21, to anchor to ground the potential of the resin portion adjacent to metal region 24. As such, fabrication of the electrical field delimiting structure according to the present invention involves no additional cost, merely alteration of the various fabrication masks. Clearly, changes may be made to the device and fabrication process as described and illustrated herein without, however, departing from the scope of the appended claims. In particular, the integrated device may be any type formed using any technique; and deep region 28 may be formed in any appropriate manner for electrically connecting metal anchoring region 24 to the substrate or any other region of the device definitely at low voltage in any condition; the term "low voltage" being intended to mean, as already stated, potentials not resulting in polarity inversion of the semiconductor material.

Claims: An integrated device (10) with a surface electrical field delimiting structure (32), comprising a semiconductor material body (11, 12) including a substrate (11) of a first conductivity type and an epitaxial layer (12) of a second, opposite, conductivity type, said semiconductor material body having a surface (19) covered with a passivation layer (18) underlying a plastic material layer (30); characterized by a deep contact region (28) having said first conductivity type extending in said semiconductor material body (11, 12) from said surface (19) to said substrate (11) and a conductive material formation (24) formed on said deep contact region in an opening (21) of said passivation layer (18) which is filled with plastic material, said conductive material formation (24) and deep contact region (28) forming electrically anchoring means (32) anchoring said plastic material layer (30) to said substrate (11). A device as claimed in Claim 1, comprising high-voltage formations (23) to be biased at a high voltage contacting said plastic material layer (30) ; characterized in that said anchoring means (32) are located close to said high-voltage formations. A device as claimed in Claim 1 or 2, wherein said semiconductor material body (11, 12) comprises active regions (14) integrating electronic components; characterized in that said electrical anchoring means (32) are not connected to said electronic components in said active regions (14) through paths along which humidity can travel. A device as claimed in any one of the foregoing claims, characterized in that said conductive material formation comprises a metal line (24). A process for fabricating an integrated device (10) with a surface electrical field delimiting structure (32), comprising the steps of: forming a semiconductor material body (11, 12) including a substrate (11) of a first conductivity type and an epitaxial layer (12) of a second, opposite, conductivity type; forming a passivation layer (18) on the surface (19) of said semiconductor material body; and forming a plastic material layer (30) over said passivation layer; characterized by the steps of: forming, in said semiconductor material body (11, 12), a deep contact region (28) having said first conductivity type and extending from said surface (19) to said substrate (11); and forming a conductive material formation (24) on said deep contact region (28) in an opening (21) of said passivation layer (18); and filling said opening (21) with plastic material. A process as claimed in Claim 5, characterized in that it comprises the step of forming high-voltage formations (23) to be biased at a high voltage contacting said plastic material layer (30) ; and in that said anchoring means (32) are formed close to said high-voltage formations. A process as claimed in Claim 5 or 6, characterized in that it comprises the step of forming at least one through opening (21) in said passivation layer (18), forming said conductive material formation (24) on said surface (19) of said body of semiconductor material (11, 12) at said at least one opening (21) and filling said at least one through opening with said plastic material. A process as claimed in any one of claims 5 to 7, characterized in that said formation of conductive material comprises a metal line (24). A process as claimed in any one of the foregoing Claims 5 to 8, characterized in that it comprises the steps of: simultaneously forming, in said epitaxial layer (12), deep junction isolating regions (13) and said deep contact region (28) extending between said surface (19) and said substrate (11); forming electronic components in said epitaxial layer (12); simultaneously forming metal regions and pads (23, 25) for electrically connecting said electronic components on said surface (19), and said conductive material formation (24) on and electrically contacting said deep contact region (28); forming said passivation layer (18) covering said surface (19); simultaneously forming, in said passivation layer (18), openings (20, 22) exposing said pads (23, 25), and one opening (21) exposing said conductive material formation (24); forming external connection elements for said contact pads (23, 25); and forming said plastic material layer (30) so as to fill said one opening (21).

Inventors: CONTIERO CLAUDIO, DIAZZI CLAUDIO, MASTROMATTEO UBALDO, MURARI BRUNO, CONTIERO, CLAUDIO, DIAZZI, CLAUDIO, MASTROMATTEO, UBALDO, MURARI, BRUNO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1994
Main classification: H01L23
IPC codes: H01L21, H01L23, H01L27